This is about the mysterious banging, clanging, construction-type sounds heard in (one or both of) the twin towers in the weeks and months prior to 9/11. It was discussed in the (Internet) film 9/11 MYSTERIES. First, famed witness William Rodriguez reported the sounds. He said they were coming from a floor at which no elevators stopped. There was no button to go there. Then, much more vividly, another guy (whose name eludes me) reported the sounds on the floor above him. (We can assume others on his floor also heard them and were discussed. He wasn't hallucinating). Out of curiosity he went to look up there and, holy cannoli, the entire floor was utterly empty; devoid of any sign of activity or anything else!

Could there have been ghosts or other invisible beings operating there?

This is obviously the most overlooked aspect of the saga. How many other witnesses are alive? This needs to be explored to the fullest.